The main pollutants from diesel engines are, apart from the very small amounts of hydrocarbons (HC) and carbon monoxide (CO), nitrogen oxides (NOx) and soot particles (PM). The soot particles are composed of a constituent which is soluble in organic solvents and a constituent which is insoluble. The soluble part comprises a large number of different hydrocarbons which are condensed or adsorbed or absorbed on the particle core. The insoluble component comprises sulfur trioxide or sulfate, carbon, abraded metal (for example iron and nickel) and small amounts of other oxides formed from additives in lubricating oil and in the fuel (for example zinc, calcium, phosphorus). Sulfur trioxide is formed by oxidation of sulfur dioxide over the catalyst as a function of temperature, noble metal loading and exhaust gas flow. A particular characteristic of diesel engines is the high oxygen content of the exhaust gas. While the exhaust gas of stoichiometrically operated gasoline engines contains only about 0.7% by volume of oxygen, the exhaust gas of diesel engines can contain from 6 to 15% by volume of oxygen.
The ratio of the various pollutants in the diesel exhaust gas to one another depends on the type of diesel engine and its mode of operation. In principle, what has been said above applies both to stationary diesel engines and to diesel engines in motor vehicles for light and heavy duties.
The permissible emissions of diesel engines are subjected to upper limits imposed by legislation. To adhere to these limits, various concepts are employed depending on the type of diesel engine and its mode of operation.
In the case of relatively low power diesel engines in passenger cars, it is frequently sufficient to pass the exhaust gas over a diesel oxidation catalyst which burns the emitted hydrocarbons, carbon monoxide and also part of the soluble organic compounds adsorbed on the soot particles. The oxidation function of diesel oxidation catalysts is designed so that although they oxidize the organic compounds and carbon monoxide, they do not convert the nitrogen oxides and sulfur dioxide into more highly oxidized species. Together with the remaining proportion of the particles, the nitrogen oxides and sulfur oxide leave the catalyst virtually unchanged. A typical representative of such catalysts is described in DE 39 40 758 A1 (U.S. Pat. No. 5,157,007).
The conversion of pollutants by means of such catalysts is strongly dependent on the temperature. In the case of carbon monoxide and hydrocarbons, the conversion of the pollutants increases with increasing exhaust gas temperature. The temperature at which a prescribed percentage, usually 50%, of a pollutant is reacted is referred to as the light-off temperature of the catalyst for the conversion of this pollutant. It is an important parameter for describing the catalytic activity of the catalyst.
Furthermore, the aging state of the catalysts has a significant influence on the degree of conversion for the various pollutants. As aging increases, the catalytic activity of the catalysts decreases. Aging can comprise damage caused by thermal overloading and/or poisoning by poisoning elements such as lead, phosphorus, calcium and sulfur, some of which are present in the fuel or are constituents of motor oil.
The catalysts have to be able to ensure adherence to particular limit values for pollutant conversion even after the vehicle has been driven for up to 150 000 miles. This requirement is usually fulfilled by over-dimensioning of the fresh catalyst. Thus, for example, it can be designed so as to be significantly larger than would be necessary on the assumption of its fresh activity, or the catalyst formulation in terms of composition and noble metal loading is adapted appropriately.
It is known that high noble metal loadings have to be used in diesel vehicles in order to be able to adhere to the emission limits even after aging because of the low exhaust gas temperatures.